[MRG+2] Unify meas_date (to use a tuple)

[larsoner]

Closes #5499.

[agramfort]

@massich please have a look

[codecov]

Codecov Report

Merging #5500 into master will decrease coverage by <.01%.
The diff coverage is 84.84%.

@@            Coverage Diff             @@
##           master    #5500      +/-   ##
==========================================
- Coverage   88.28%   88.27%   -0.01%     
==========================================
  Files         360      360              
  Lines       66892    66897       +5     
  Branches    11317    11320       +3     
==========================================
- Hits        59053    59051       -2     
- Misses       5020     5024       +4     
- Partials     2819     2822       +3

[massich]

IMHO at some point, we should also rename this _handle_meas_date function in favor of something more meaningful like _absolute_time or _meas_date_in_absolute time.

mne-python/mne/annotations.py

Line 272 in 613df87

def _handle_meas_date(meas_date):

but this can be done in a subsequent PR.

[agramfort]

@massich please merge when CIs are green.

[sappelhoff]

Looks great, thanks @larsoner